The present invention relates to a riser pipe construction for a drilling connection from a drilling vessel to a valve previously provided on a sea floor, comprising a riser pipe through which drilling means can be passed by means of which an oil well can be drilled, as well as pressure pipes extending along the riser pipe to operate the valve, and floating elements disposed around the riser pipe and the pressure pipes to limit the load on the drilling vessel, the riser pipe and the pressure pipes being built up from modules capable of being coupled together, the floating element being formed by at least one steel tubular chamber closed so as to be gastight, disposed around a module and firmly connected thereto.
Such a riser pipe construction is known from e.g. U.S. Pat. No. 3,354,951 or U.S. Pat. No. 3,330,340. In the known construction, the chambers form a protection against possible damage to the pressure pipes and perhaps to the riser pipe during the lowering of the riser pipe into the sea from the drilling vessel, while the air in these chambers already imparts a certain floating power to the riser pipe.
U.S. Pat. No. 3,378,067 describes an under water well head with a buoy from which water can be expelled by means of high pressure gas in a capsule. With this buoy a connection head on the sea bottom can be raised or the location of the connection head can be marked.
U.S. Pat. No. 4,099,560 discloses a riser pipe construction having floating elements formed by open tubular members from which water can be expelled by releasing air from a pressurized container.
The known construction is used during the drilling of oil wells under the sea floor, after they have already been located during exploratory drillings and after a connecting body with a valve in the form of a xe2x80x9clower riser marine package (LRMP)xe2x80x9d has been placed on the sea floor by means of a robot. From a drilling vessel, a riser pipe construction is passed down stepwise by mounting individual modules together in each step and lowering them into the sea from an opening at the bottom of the drilling vessel, and by means of a robot connected to the connecting body on the sea floor. Subsequently, the valve is opened by means of the pressure pipes and the drilling means are passed through the riser pipe to drill the oil well. These drilling means comprise a drill head and a narrower pipe passed through the riser pipe. Then the drilled oil well must be closed. However, as a result of the drilling of the oil well, gases and oil can be released and leak via the space between the narrower pipe and the riser pipe. This leaking occurs at pressures of from 200 to 300 bar, so that oil and gas may rise with tremendous force and constitute a danger on the drilling vessel. To prevent this, the riser pipe is filled with mud to apply a counter pressure slightly higher than the leaking gas/oil pressure.
After the oil well has been closed, the riser pipe construction may then be coupled off and moved up into the drilling vessel. The mud contained in the riser pipe is released into the sea. Although at a later time a drilling platform for the oil extraction may take the place of the drilling vessel, there is an increasing tendency to extract the oil from the drilling vessel, in which case an oil pipe must be passed through the riser pipe. In that case, however, the drilling vessel must be kept in place, which may be done by means of anchor cables or, if the sea is too deep, by using an engine.
In practice, a riser pipe is built up from steel pipe modules which often have a length of 75 feet (about 23 m) and an inner section of 19 inches (about 48 cm). In practice, the installation of such a riser pipe is attended by different problems. First, the water pressure constitutes a problem with relatively long riser pipe constructions. This problem particularly becomes apparent when the sea floor is at a depth of more than 2000 m. It must be realised that at present about 20% of the estimated world oil supply is exploited at a depth of less than 2000 m, while about 70% of this supply is at a depth of from about 2500 to 4000 m. To exploit this oil supply, because of the necessary large steel plate thickness which a riser pipe must have for this depth to resist the water pressure and, consequently, the heavy weight thereof, there must be built drilling vessels other than are in use at present. Consequently, to enable partial compensation of the weight of the riser pipe, there are already used floating elements disposed around the riser pipe and the pressure pipes. The known floating elements consist of plastic blocks, in particular of polystyrene, which are filled with air. However, the maximum water depth at which these floating elements can be used is about 2200 m. At greater depths, it turns out that these floating elements are pressed together or implode and air escapes therefrom, with the result that the floating power decreases and the compensation for the weight of the riser pipe becomes insufficient. An even heavier hoisting construction and an even larger design of the drilling vessel are then necessary. When a riser pipe is passed down stepwise from the opening at the bottom of the drilling vessel, the riser pipe, certainly when the first modules have been mounted, will be carried away by the water current. Then the riser pipe nearly always comes in contact with the bottom of the hull, which may easily damage the floating elements and the pressure pipes in particular. If damage has been caused to the floating elements, the floating power is further decreased with the attendant above-mentioned disadvantages. If damage has been caused to the pressure pipes, the whole riser pipe must be moved up again to enable repair. Negligence in this respect leads to an attack on the environment because oil may find its way from a pressure pipe into the sea. Moreover, the costs involved in such repairs are extremely high, particularly because repair of the riser pipe constructions that are in use at present must be carried out ashore, which also causes a lot of problems from an insurance standpoint.
It is an object of the invention to remove, at least substantially reduce, these disadvantages.
According to the invention, the riser pipe construction as defined in the opening paragraph is therefore characterized in that the tubular chamber is filled under elevated pressure with a medium.
By filling the chambers under elevated pressure with a medium, for instance air, in particular up to a pressure of the order of 100 bar, the load on the riser pipe and the floating elements decreases. At a depth of about 2000 m, a pressure of about 200 bar is exerted on the riser pipe construction. At a pressure of about 100 bar in the chambers of the floating elements, there results a pressure of 100 bar on the outer wall of the riser pipe construction. When during the drilling of the oil well gas and oil leak and the riser pipe is filled with mud at a pressure of the order of 300 bar, the pressure on the wall of the riser pipe is reduced by the pressure in the chambers of about 100 bar to about 200 bar. Through the construction according to the invention, it further becomes possible to carry out repairs on the drilling vessel itself, thus saving transport costs.
By providing the tubular chambers with partitions disposed substantially radially with respect to the riser pipe, they can be reinforced such that the plate thickness of the steel chambers can be reduced. This leads to a further decrease of the load on the drilling vessel.
When one of the tubular chambers becomes defective and lets in water, it is favorable if these partitions are so strong that they can take up the resulting pressure increase on the wall of the riser pipe. This means that there may be modules which need not by any means be provided with chambers disposed around the riser pipe. In that case, the floating power of the riser pipe construction decreases; the whole construction, however, is so dimensioned that a decrease of the floating power by about 10% is still acceptable. The invention therefore also relates to a riser pipe construction for a drilling connection from a drilling vessel to a valve previously provided on a sea floor, comprising a riser pipe through which drilling means can be passed by means of which an oil well can be drilled, as well as pressure pipes extending along the riser pipe to operate the valve, the riser pipe and the pressure pipes being built up from modules capable of being coupled together, characterized in that the riser pipe comprises substantially radially disposed partitions. Such a riser pipe may of course comprise floating elements which are disposed around the riser pipe to limit the load on the drilling vessel, and which are formed by at least one steel tubular chamber closed so as to be gastight, disposed around a module and firmly connected thereto.
As stated before, the riser pipe may be filled with mud to provide a counterpressure against leaking oil and gases from the connecting body. When the connecting body is closed again and the riser pipe must be moved up, this mud, which may perhaps be fully permeated by oil and gases, finds its way into the sea, which is an undesirable situation from an environmental standpoint. Hence, according to another aspect of the invention, the riser pipe comprises at least one opening closable by a valve, and capable of being put into communication with a relevant pipe extending upwards through the floating elements. Via this pipe, at least part of the mud can be sucked up before the riser pipe is moved up. Preferably, there are three of such pipes. To facilitate the moving up of the mud, this connection includes a pump placed within a chamber. Because the power of a pump, particularly because this pump must be placed in a chamber of limited space, cannot be chosen too high, it proves to be a right choice if the relevant opening is provided about halfway the total length of the riser pipe.
In a concrete embodiment according to the invention, the modules are provided at the ends with at least one flange part and can be coupled together through this flange part, while a tubular chamber extends along a module in the longitudinal direction to near the relevant flange part, on the one hand, and, on the other hand, the relevant connecting place at the flange part of a module to be coupled. To protect the coupling part of the modules, a covering element is preferably disposed between the tubular chambers around two modules coupled together. In particular, the tubular chambers have a cylindrical shape and the same diameter, while the covering element also has a cylindrical shape and the same diameter as the tubular chambers. Consequently, the riser pipe has, over its full length, a cylindrical shape with a fixed diameter, so that it can be lowered into the sea from the drilling vessel by means of guide rollers.
To further limit the weight of the riser pipe construction, special steel types may be used. Thus, for instance, it is possible that the tubular chambers are manufactured from steel having a plate thickness of the order of from 10 to 25 mm, preferably about 18 mm, and a yield strength of at least 800 N/mm2, preferably about 1100 N/mm2. Such a steel type is commercially available under the name of Weldox 1100 steel from the firm of SSAB of Oxelxc3x6sund of Sweden. The tubular chambers and covering elements can easily resist a water pressure up to a depth of at least 3500 m, while yet the total weight of the riser pipe construction as such can be kept limited so as to enable working with the existing drilling vessels.
Each vessel can take a maximum of tonnage (payload), so that a light design is very important. If the drilling vessel cannot take in enough riser pipe construction parts, particularly for a greater depth, the further riser pipe construction parts must be conveyed by a separate transport vessel. Drilling more deeply generally means drilling at a greater distance from the coast and, consequently, higher transport costs. The above-mentioned measures taken to limit the load on the ship by means of a lighter design of the riser pipe construction therefore lead, particularly during the drilling at a greater depth and farther from the coast, to substantial savings in costs.
The invention also relates to module for a riser pipe construction.